Broadcast channels in the FM band (e.g., about 76 MHz to 108 MHz) are often transmitted with an FM stereo multiplex (MPX) format where the MPX signal includes left (L) channel and right (R) channel information that can then be used by an FM receiver to produce stereo audio outputs. In particular, the FM stereo MPX signal format includes L+R (left-plus-right) information, L−R (left-minus-right) information and a 19 KHz pilot tone. With respect to the center frequency of the broadcast channel, the L+R information lies in a band between 30 Hz and 15 KHz from the center frequency in the broadcast channel. The L−R information lies in two bands on either side of 38 KHz from the center frequency of the broadcast channel, namely a first band between 23 KHz and 38 KHz and a second band between 38 KHz and 53 KHz. And the pilot tone sits at 19 KHz from the center frequency of the broadcast channel. Most FM receivers will produce a full stereo output by generating a left (L) channel audio output by adding the L+R and L−R signals ((L+R)+k*(L−R)=2L when k=1) and a right (R) channel audio output signal by subtracting the L+R and L−R signals ((L+R)−k*(L−R)=2R when k=1). If a mono output is desired or selected, most FM receivers will drive k to 0 in order to pass the full (L+R) signal to both the left (L) channel and the right (R) channel. In addition, varying degrees of a blend from stereo to mono can also be provided by adjusting or varying k from 1 to 0 to provide the desired level of a blend from stereo to mono.
FM broadcast band receivers can suffer from noise due to strong nearby blockers and/or other sources or conditions, and this noise or interference will often show up as static in the stereo audio output for the tuned FM channel. Techniques have been proposed before to mitigate this stereo noise. One prior technique is to use a receive signal strength indicator (RSSI) to blend the audio output from stereo to mono when the RSSI indicates poor signal strength. Another prior technique is to use a signal-to-noise ratio (SNR) measurement of the incoming signal to blend from stereo to mono when the SNR is low. The SNR measurement can be made, for example, by analyzing the amplitude modulation in the received FM broadcast signal. In addition, other solutions have proposed blending to mono based upon an analysis of variations in the FM pilot tone and/or an analysis of high frequency components that are above the frequencies for the FM information in the tuned signals. While these techniques provide some success in mitigating stereo noise, improved techniques are needed.